This invention relates to dewatering of a lime mud and particularly to dewatering of lime mud in the chemicals recovery cycle of a kraft pulping process.
Processes of preparing cellulosic pulps including kraft pulping process are within the skill in the art for instance as discussed in Casey, Pulp and Paper; Chemistry and Chemical Technology, 3rd ed., vol. 1, (1980) especially pages 291-491 and 504-567. In chemical pulps, the wood or other cellulose source is advantageously separated into pulp with the help of chemicals. Two principal chemical pulping methods are in use: soda process and kraft process. The soda process utilizes a strongly alkaline solution of sodium hydroxide to digest wood chips. Kraft pulping process utilizes sulfate materials (reduced to sulfites in the furnace) and hydroxides. The kraft pulping process is widely used these days for producing pulp for subsequent processing to produce paper for instance as discussed in Gary A. Smook, xe2x80x9cHandbook for Pulp and Paper Technologistsxe2x80x9d, Second Edition, Angus Wilde Publications, pp. 74-83 and 133-162 (1992).
In this process, wood chips are cooked (digested) under conditions of heat and pressure using xe2x80x9cwhite liquorxe2x80x9d containing sodium hydroxide (NaOH) and sodium sulfide (Na2S) to release cellulose fibers from other components such as lignin. After digestion, fibers are commonly released under pressure into a tank in a process referred to as blowing or blowdown. Then the pulp is washed to remove spent chemicals, lignin and other organic chemicals. The liquid removed from the pulp is referred to as xe2x80x9cblack liquorxe2x80x9d and contains about 25 percent dissolved solids. The black liquor resulting from the washing stage is concentrated by evaporation to a desired concentration and burned to reclaim the inorganic chemicals and provide fuel value. The organic materials are advantageously incinerated to yield an inorganic smelt of sodium carbonate (lime mud) and sodium sulfide. The resulting inorganic smelt is dissolved to form xe2x80x9cgreen liquorxe2x80x9d. Clarified green liquor is reacted with lime (CaO) in a slaker (causticizer). This produces the white liquor (which is reused in the digestion step) and lime mud. The lime mud is then recalcined in a heated lime kiln to recover lime (CaO) which is used in a slaker.
An important step in the processing of lime mud is lime mud dewatering. This is typically done using a suitable filtration means such as vacuum drum filter. Typically, a rotary vacuum drum filter is used for dewatering lime mud and washing it just prior to its entrance into the lime kiln. Lime mud from storage is diluted to 25-35 percent solids and pumped to the xe2x80x9cprecoatxe2x80x9d filter. This filter operates at 15-20 inches of vacuum (about 9 psia). The drum is covered with a screen made of stainless steel or plastic fiber (typically 150 mesh). A cake of lime mud builds up on the screen as the drum turns, and a doctor blade is fixed at a distance of xe2x85x9c-⅝ inch (about 0.94-1.56 cm.) from the screen. Consequently, a layer of lime mud remains on the screen continually and acts as the filter medium for the lime mud. Thus the name xe2x80x9cprecoatxe2x80x9d filter. This xe2x80x9cprecoatxe2x80x9d enhances the filter""s ability to remove fine particles during the filtration process. During the filtration, as the lime mud builds up, the doctor blade removes it and the dewatered lime mud falls onto a screw feeder which transports it to the feed end of the kiln. Dewatered lime mud is typically about 65-75 percent solids. The temperature at the pre-coat filter is important. Best results are seen with temperatures of about 70xc2x0 C., while cold temperatures can reduce filter capacity by 10 percent or more.
However, many filters are not efficient and the use of dewatering additives would be desirable to facilitate removal of water from and improve filtration of the lime. The use of dewatering additives in the lime mud processing would not only improve filtration of lime mud but would also result in less energy required for heating the lime kiln to convert lime mud to lime.
The main benefit of using lime mud dewatering additives would be the reduction of the water content of lime mud exiting the filter. Additional benefits may be any of the following: reduction of fuel consumption in the lime kiln resulting in the energy savings; reduction of formation of xe2x80x9cringsxe2x80x9d and xe2x80x9cballsxe2x80x9d in the kiln (better water removal results in greater removal of the water-soluble salts responsible for these formations in the kiln); reduction of sulfur stack emissions from the process (much of sulfur is present at this point in the process as sodium salts; better dewatering results in more efficient sulfur removal); increased lime mud filter runability (a drier filter cake results in less plugging of the screw feed that transports dewatered lime mud to the kiln resulting in lower maintenance and less needed cleanup).
Thus, there is a clear need in the cellulosic pulp industry for an additive which will enhance dewatering of lime mud.
It has now been discovered that the use of alkyleneamines improves reduction of the water content of lime mud exiting the filter and improves on one or more of the aforementioned benefits.
The present invention concerns an improved process for dewatering lime mud wherein the improvement comprises adding an effective amount of an alkyleneamine to lime mud prior to filtration.
In another aspect, the present invention concerns an improvement in a kraft pulping process wherein lime mud is filtered to remove excess of water, the improvement comprising the addition of an effective amount of an alkyleneamine to lime mud prior to filtration.
In yet another aspect, the present invention concerns a lime mud dewatering composition comprising lime mud and an effective amount of an alkyleneamine.
This invention is applicable to any lime mud and particularly to lime mud in the spent chemicals recovery cycle of the kraft cellulosic pulping process.
The term alkyleneamine is used to mean an amine having at least one alkyleneamine unit or repeating alkyleneamine units such as, for example, ethyleneamine, propyleneamine, and butyleneamine. The preferred alkyleneamine is ethyleneamine, that is, an amine having at least one ethyleneamine unit or repeating ethyleneamine units. An ethyleneamine unit is xe2x80x94(CR2xe2x80x94CR2xe2x80x94NHxe2x80x94)xe2x80x94 where R is H or an alkyl (straight, branched or cyclic) group, preferably H, but if alkyl of from about 1 to about 10 carbon atoms. Ethyleneamines have at least two amine groups, which groups are primary or secondary amine groups; tertiary amine groups are optionally also present. Thus, ethyleneamines include ethylenediamine (EDA), diethylenetriamine (DETA), triethylenediamine (TEDA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), piperazine (PIP), aminoethylpiperazine (AEP), ethyleneamine mixtures such as mixtures of ethyleneamine oligomers having an average molecular weight of about 250-500 commercially available from The Dow Chemical Company under the trade designation Ethyleneamine E-100 (E-100), and other mixtures thereof. In the case of ethyleneamines having isomers, one isomer or a mixture of isomers is suitably used in the practice of the invention. It is preferred that the ethyleneamine be soluble in the aqueous lime mud composition; therefore, the molecular weight or average molecular weight in the case of a mixture of the ethyleneamines is preferably sufficiently low to retain solubility in the aqueous lime mud composition, preferably in lime mud water slurry. More preferably, the molecular weight or average molecular weight of the alkyleneamine is from about 50 to about 1000, more preferably from about 100 to about 500, most preferably from about 200 to about 500. Among ethyleneamines, ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and Ethyleneamine E-100 ethyleneamine oligomers mixtures are preferred with ethylenediamine, diethylenetriamine, triethylenetetramine, and tetraethylenepentamine more preferred and Ethyleneamine E-100 ethyleneamine oligomers mixtures most preferred.
Conditions of use are not critical to the invention. However, dewatering is expected within the art to be more efficient at elevated temperatures than at temperatures at or below room temperature. Best dewatering results are seen with temperatures of about 70xc2x0 C. to about 90xc2x0 C. while cold temperatures can reduce filter capacity by 10 percent or more. A temperature of about 77xc2x0 C. is conveniently used in dewatering of lime mud.
The alkyleneamines are used in any amount effective to enhance dewatering of lime mud. The concentration of the alkyleneamines which has been found to be effective for enhancing dewatering lime mud is typically in the range of from about 10 to about 10,000, preferably from about 100 to about 5,000, most preferably from about 500 to about 3,000, parts per million by weight (ppm) based on the weight of lime mud composition.
The alkyleneamine is conveniently added to the lime mud composition prior to the filtration thereof. Typically, the alkyleneamine is added to lime mud coming out of storage or through the water showers that are directed upon the filter cake on the lime mud filter.
The use of alkyleneamine dewatering additive can increase the percent of solids by 3-5 percent in the lime mud having solids content above 70 percent and by more than 10 percent in the lime mud having solids content of 60-65 percent.
The following examples are offered to illustrate but not limit the invention. All ratios, percentages and parts are by weight unless otherwise indicated.